The present invention relates to transistorized variable gain amplifiers particularly suited for adjustable gain control and automatic gain control.
Typically, amplifiers achieve variable gain control by coupling a pair of gain stages where their outputs are connected oppositely and both gain stages share the same bias current. The bias current of both gain stages are then differentially controlled by a pair of in-series transistors. This topology is termed a Gilbert cell. Amplifiers exploiting a Gilbert cell can provide wide variations in gain. However, because of in-series control circuitry, the minimal rail-to-rail voltage is higher for the typical variable gain amplifier when compared to a conventional fixed-gain amplifier. Variable gain amplifiers exhibiting this higher rail-to-rail voltage inherently consume more power than the similarly sized fixed-gain amplifiers. Not surprisingly, the power consumption produces a practical need for an extra or enhanced power supply beyond that nominally required for conventional fixed-gain amplifiers.
U.S. Pat. No. 5,418,494, issued May 23, 1995, to G. Betti, et al., and assigned to SGS-Thomson Microelectronics, S.r.l., discloses a variable gain amplifier with low power supply. FIG. 1 shows a variable gain amplifier with a fixed-gain amplifier 10, a variable gain amplifier 20, a gain control and stabilizing variable current generator 30, and a current-to-voltage converter 40. Due to the parallel configuration of fixed-gain amplifier 10 and variable-gain amplifier 20, the amplifier can be operated with a low power supply. However, in order to compensate DC current variation, the amplifier employs an in-parallel gain control and stabilizing variable current generator 30. This compensation circuitry can introduce additional parasitic effects that result in limiting the operational bandwidth. Moreover, due to the coupling of the three circuitries 10, 20 and 30, the isolation between the voltage inputs VS and control voltage VREF1 is degraded. This mediocre isolation can cause serious stability problems when the variable amplifier is used in an automatic-gain-control (AGC) loop.
The present invention is embodied as a circuit for a variable gain amplifier that uses two differential gain stages with independently adjustable bias currents. By changing the bias currents of the gain stages, the overall gain and phase of the amplifier can be adjusted over a wide range. Neither in-series nor in-parallel circuitry is required to implement or perform gain control and thus the present invention obviates the need for relatively high rail-to-rail voltage. In addition to minimal part requirements for mechanization, the present invention features low power supply requirements while maintaining a wide operational bandwidth.
Because gain control in the present invention is performed by directly adjusting the bias currents of the gain modules with a minimal number of components required, this economy also eliminates the introduction of parasitic components to the gain modules. By the economy of components, particularly parasitic components, the bandwidth of the overall amplifier is not adversely affected by the direct gain control of the present invention. Moreover, without any extra DC compensation circuitry, the variable-gain amplifier of the present invention provides excellent isolation between the RF signals and the DC control signal thus increasing the system stability as used in an automatic-gain-control (AGC) loop.